Method and apparatus for measurement of roughness and hardness of a surface

ABSTRACT

Light is focused on an interface between a transparent probe and a surface to be analyzed. The probe is lowered onto the surface using a computer-controlled actuator. Light reflected from the surface of the probe which is closest to the surface and the surface itself recombines, producing interference effects from the spacing between the probe and the surface over a 2-dimensional area. Since the shape of the probe in known beforehand, the profile of the surface can be readily calculated from the 2-dimensional measurement of the spacing between the probe and the surface. This surface profile indicates the roughness of the surface. The surface hardness and other surface properties can be measured by pressing the probe onto the surface. The contact load between the probe and the surface is detected by a load cell. The force with which the probe is being pressed onto the surface is measured using the load cell. The surface profile can continue to be measured interferometrically while it is being deformed by the probe. The measurement of surface deformation as a function of contact pressure allows the measurement of surface hardness while causing a minimum amount of damage to the surface.

This application is a division of application Ser. No. 07/781,820; filedOct. 23, 1991; issued as U.S. Pat. No. 5,280,340 on Jan. 18, 1994.

BACKGROUND OF THE INVENTION

The rapid increase in the operating speed of computers has led tosignificant demands on both data storage capacity and access speed. Thehard disk drive has been steadily evolving and continues to offer acost-effective solution to both capacity and speed requirements.Increases in data storage densities and miniaturization of disk driveshave enabled even small portable computers to have access to largeamounts of on-line storage.

In disk drives, head-to-media speeds are such that an air bearing isgenerated between the head and the disk. At increased speeds the airbearing increases. Thus, without means to counter the tendency toincrease the head/media spacing, losses will occur.

Manufacturers of hard disk drives typically measure the flying height ofall head/gimble assemblies before assembling them into drives in orderto avoid reworking drives after assembly when they do not meetspecifications. While head/media spacings in excess of 1 micron can bemeasured using monochromatic fringe counting techniques, spacings below1 micron are measured using white light interferometry due to itsgreater resolution in the range of 250 to 750 nm. Constructive ordestructive interference results in the generation of different colorfringes which are compared to a Newton's color chart or analyzed byspectrometer. This technique is the current industry standard, however,at spacings of less than 150 nm, the colors wash together and cannot beinterpreted with reasonable accuracy.

Various other techniques have been developed for the measurement ofsmall spacings, however, there methods are still incapable of accuratelymeasuring spacing down to contact. One example is U.S. Pat. No.4,593,368 by Fridge et al. This patent describes the use of acomputerized spectro-photometer to analyze the color of white lightfringes produced at the interface of two surfaces, one of which istransparent, when subject to broad-band illumination. This measuringsystem and technique has the following disadvantages: 1) At very lowspacing (less than λ/5) no distinct colored fringes are produced.Therefore, at this small spacing, relatively small changes in lightintensity as a function of wavelength are measured by the computerizedspectro-photometer. Since the measurable change in intensity as afunction of wavelength (the color) is greatly reduced at spacings belowλ/5, the signal-to-noise ratio of the measurement greatly decreases forsuch small spacings. 2) The spectro-photometer employed requires 0.05seconds to acquire the intensity data for the spectrum of light beingused for the measurement. The lengthy time required for data acquisitionprecludes dynamic measurement of spacing above 10 Hz.

In a method disclosed by Tanaka et al. (U.S. Pat No. 4,630,926) aninterferometer is used to dynamically measure head/disk spacing. A xenonsource with a monochrometer produces monochromatic light which isdirected over the length of a slider which is inclined such that thespacing between a clear glass disk and the slider varies by more thanλ/4. In such a case where the slider is incident with respect to thedisk and the spacing varies by more than λ/4, at least one maxima andone minima of interferometric fringe intensity occurs. Tanaka et al.teach that at the maximum and minimum (extrema) of fringe intensity,two-beam and multi-beam interferometric theory yield the same spacing.Therefore, at the extrema, the simpler two-beam theory is used. Tanakaet al. also vary the wavelength of the light being used in order to 1)get fringe extrema and, therefore, spacing measurement at differentpoints along the slider; and 2) verify which order fringe is beingdetected.

The system of Tanaka et al. is limited in that it cannot measure spacingbelow λ/4 of the monochromatic light being used and it is too slow tomeasure air bearing resonances. Tanaka et al.'s system is clocked at afrequency of 15.8 kHz making it incapable of measuring typical airbearing resonances of 20 kHz or more.

Another to measure slider/disk spacing is disclosed in Ohkubo et al.'spaper "Accurate Measurement of Gas-lubricated Slider Bearing SeparationUsing Visible Laser Interferometry" which was distributed as paper87-Trib-23 by the American Society of Mechanical Engineers. This paperdescribes the basis of operation for the FM 8801 Laser-Based FlyingHeight Measuring System which is sold in the U.S.A. by ProQuip, Inc.,Santa Clara, Calif. As described in Ohkubo et al.'s paper, the systemuses a HeNe laser source. The beam from the laser goes through abeamsplitter where part of the beam is directed toward a referencephotodetector which detects variation in the intensity of the lasersource. The remaining part of the laser beam goes through a beamexpander then through a lens which focuses the illumination onto theslider/glass disk interface. This illumination causes interferencefringes which are focused onto a second measurement photodetector usedfor measuring intensity of the fringes. The measurement and referencesignals from the two photodetector are sent through amplifiers then intoa divider circuit such that the interference signal is normalized to theinput laser intensity. From the divider, the signal is sent an A/Dconverter to a desk top computer for processing. The desk top computerdigitizes interferometric intensity while the disk changes from a highspeed to a low speed.

During the change of disk speed, Ohkubo et al. show flying height todecrease by more than λ/2. Since the flying height changes, theinterferometric intensity varies enough to detect at least one maximumand one minimum fringe intensity. These maximum and minimum fringeintensities are recorded for reference. With the reference maxima andminima of fringe intensity, multi-beam interferometric theory is appliedto determine spacings from intensity. However, since for a monochromaticinterferometric intensity is a periodic function of spacing, the "fringeorder" must be know to finally determine the spacing. This "fringeorder" is precisely defined as the interval of spacing from n/4λ to(n-1)/4λ for n=1, 2, 3. Given an interferometric intensity, and amaximum and minimum fringe intensity for reference, the fringe order nmust be determined in order to calculate spacing from theinterferometric theory. According to Ohkubo et al.'s paper, the fringeorder is determined by landing the slider on the disk by reducing diskspeed while monitoring interferometric intensity. The fringe order canbe determined by counting the number of times that the interferometricintensity rises to the maximum or falls to the minimum while the spacingis being reduced from the measurement point to the minimum spacing whichis assumed to be the fringe order where n=1.

The Ohkubo et al. system has the following disadvantages: 1) the slidermust have a design such that the flying height will increase to aboveλ/2 simply by changing disk speed; 2) the slider must be landed on theglass disk to determine the fringe order for the spacing calculation;and 3) at the points where the fringe intensity is a minima or maxima,the slope of the interferometric intensity/spacing curve becomes zero.At these points, the noise in electronic intensity measurement causes alarge error in spacing measurement relative to the other spacings whichare not directly on the fringe maximum or minimum.

The above-identified disadvantages may cause the following problems: 1)new sliders with a new geometries designed for very low flying heightmay not fly as high as λ/2, even at very fast disk speeds, so the Ohkuboet al. technique will not work for such; 2) landing the slider on thedisk (required to determine the fringe order) may cause some damage tothe air bearing surface of the slider. The possibility for damage to theair bearing surface during test is highly undesirable since manymanufacturers test every slider assembly to insure proper flying height;and 3) the relatively high error in spacing measurement at fringe maximaand minima is hidden by "intensity correction" and data "smoothing".These procedures can introduce additional errors into the spacing whichis finally calculated.

As magnetic recording technology continues to improve, slider flyingheights should continue to decrease to below 100 nm. Also, somemanufacturers are beginning to use fluid in the gap to permit smallerspacings. These, too, must be measured. The invention disclosed in thispatent is intended to measure such flying heights, statically anddynamically, without having to land the slider on the disk, or have thespacing increase above λ/2 by only changing disk speed.

The method of intensity calibration and fringe order determinationdisclosed in this patent can also be applied to other measurements wherespacing is decreased to the point of contact, in particular, amicro-hardness tester using a transparent probe could be implementedwith interferometric measurement of spacing between the surface and theprobe using this method of interferometric intensity calibration.

SUMMARY OF THE INVENTION

It is an advantage of the present invention to provide a device formeasuring static or dynamic spacing between a transparent article and areflective surface spacing where the spacing is air or a fluid (down tocontact).

Another advantage is to provide a system for measuring flying heightwhich does not require landing of the slider on the disk to determinefringe order.

It is a further advantage of the present invention to provide a flyingheight tester which uses a calibration procedure to determine maximumand minimum intensity of interference fringes yet is insensitive tochanges in illumination intensity or surface reflectivity.

It is still another advantage of the present invention to provide adevice capable of measurement of disk surface roughness for an entiredisk.

In an exemplary embodiment, the dynamic flying height tester uses amercury arc lamp light source to provide three distinct wavelengths oflight so that three separate interference fringes are generated. Lightfrom the mercury arc lamp is directed substantially normal to thesurface of a transparent disk, through the disk and onto the slider onwhich a magnetic head is mounted. The light reflected from the sliderand from the surface of the disk closest to the slider is combined andspectrally analyzed for constructive and destructive interference ateach of the three wavelengths. The spectral analysis is accomplished bya detector assembly which includes wavelength discriminatingbeamsplitters, a filter for each individual wavelength to be measuredand a high speed photodetector for each wavelength. The microscope isalso connected to a video monitor for visual monitoring of the fringepattern.

A calibration procedure involves measurement of the intensity of allcolors while partially unloading the head to determine the maximum andminimum intensity of the fringes for each color being used and toidentify the correct fringe orders of the interference patterns. Thepartial unload for calibration is implemented by rotary head unloadwhich is used to move the head away from the detection assembly by avery small distance, on the order of 0.25 μm. In addition to thedemonstrated rotary unload mechanism, any mechanical device which causesthe spacing between the head and disk to vary by a quarter wavelength ormore can be used to cause the spacing change required for calibration.

Since some parameters such as disk speed or head/disk relative positionmay be changed after the calibration but prior to the desiredmeasurement, a "follow" trace is made to determine the fringe orders ofthe measurements trace. The follow trace consists of a spacingmeasurement trace. The follow trace consists of a spacing measurementwhich is made during the time any changes occur between the calibrationtrace and the measurement trace. The follow trace is often required totrack the fringe order when changes are made to the head/disk system inbetween the calibration procedure and the measurement.

Once the calibration trace and follow trace are analyzed, the absoluteinterferometric intensity reference is known, and the fringe orders forthe measurement are known, Therefore, measurement of flying height isaccomplished by comparing measured intensity with the theoreticalintensity versus spacing relationship. The measurement of spacing can bedisplayed as spacing versus angular position of the disk or spacingversus time. Also, parameters such as average spacing, maximum spacing,minimum spacing and others can be calculated from the dynamic spacingdata and logged for future reference.

An alternative light source can be a plurality of lasers where eachlaser emits a different wavelength light, the wavelengths preferablybeing relatively short to enable measurement of small spacings.

BRIEF DESCRIPTION OF THE DRAWINGS

Understanding of the present invention will be facilitated byconsideration of the following detailed description of a preferredembodiment of the present invention, taken in conjunction with theaccompanying drawings, in which like reference numerals refer to likeparts and in which:

FIG. 1 is a diagrammatic view of the flying height testing assembly ofthe present invention;

FIG. 2 is a diagrammatic view of the detector assembly;

FIG. 3 is a diagrammatic view of the assembly with an alternate lightsource;

FIGS. 4a-4d are a series of plots of spacing versus angle of rotation;

FIG. 5 is a plot of spacing versus time;

FIG. 6 is an alternate configuration of the testing assembly formeasurement of disk surface roughness;

FIGS. 7a and 7b are diagrammatic view of the apparatus for attaching thehead assembly to the load/unload arm;

FIG. 8a is a diagrammatic top view of the rotary unload mechanism, FIGS.8b and 8c illustrate the load and unload positions, respectively;

FIG. 9 is a diagrammatic view of an alternate embodiment in the form ofan interferometric surface roughness/surface hardness measurementinstrument; and

FIG. 10 is a plot of intensity versus spacing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As illustrated in FIG. 1, light source 2 is directed through microscope3 which comprises condensing optics 4, beamsplitter 6 and objective 8. Aportion of the incident light split by beamsplitter 6 is directedthrough glass disk 10 and is internally reflected off the lower surfaceof the glass disk 10. Another portion of the light is reflected byslider 12. The reflected light is redirected through the objective lensand the beamsplitter 6 into detector assembly 14. The recombination ofthe light reflected by the lower surface of the disk 10 and the surfaceof slider 12 results in the generation of interference fringes, with oneinterference fringe being generated for each individual wavelength oflight which has been selected by filters within the assembly. A camera18 sends a visual image of the fringe patterns to video monitor 22. Thephotodetectors within the detector assembly 14 convert the fringes'intensity into electrical signals which are then converted to digitaldata by A/D converter 24. The digital data are then processed atprocessor 26.

Disk 10 is mounted on spindle 52 which is attached to a variable speeddrive capable of achieving speeds in excess of 7200 rpm. In the firstembodiment illustrated in FIG. 1, the disk 10 is transparent glass orquartz. The slider 12 is attached to rotary load/unload arm 17, which isfurther illustrated in FIG. 8a, by clamping suspension 13, illustratedin FIG. 7, onto the end of arm 17 at a given location. Rotaryload/unload arm 17 is drive by rotary motor 7. Slider 12 is thenpositioned under the disk 10 so that light focused by objective 8 passesthrough disk 10 to be reflected from slider 12 and from the bottom edge51 of disk 10.

In an alternate embodiment shown in FIG. 6, the light source 2' andmicroscope assembly 3' are positioned relative to the arm and drive sothat the slider 12' is between the microscope assembly 3' and disk 10'.Slider 12' is transparent, permitting glide height measurement of anactual magnetic disk 10'. Such a configuration provides measurement ofdisk surface roughness to identify disk asperities for mapping ofasperity heights.

Rotary load/unload arm 17 pivots in such a manner that the slider 12moves along a line following the light path exiting from objective 8 forat least part of the unload motion. This allows the light to be focusedon the same spot on the slider 17 during the calibration procedure,which is described below. The purpose of this movement is to scan atleast fringe maximum and minimum for each color used by increasing thehead/disk spacing. Only a small spacing change required, on the order of0.25 μm, to measure a maximum and minimum intensity for each colorfringe to enable calibration. The remainder of the load/unload motion isnot critical to measurement functions.

FIG. 2 illustrates the elements of detector assembly 14. A firstbeamsplitter cube 30 reflects light at 436 nm toward absorption pinholemirror 29 and 436 nm interference filter 31 to photodetector 32.Beamsplitter cube 30 is a dichroic beamsplitter made from BK7. Theentrance face has an anti-reflection coating. The interface is coatedfor 85-95% reflectance at 435.8 nm and 85-95% transmittance at 546 nmand 580 nm. The portion of light which passes through beamsplitter cube30 passes through reflective pinhole mirror 33 to beamsplitter cube 34which reflects light at 546 nm toward 546 nm interference filter 35 anddetector 36. Beamsplitter 34 is also dichroic and made from BK7. Itsinterface coating is 90-100% reflective at 546 nm and 90-100%transmissive at 580 nm. Each of the interference filters arecommercially available filters which transmit light at the chosenwavelength with a bandwidth of 10 nm. The photodetectors are siliconavalanche photodiodes with spectral response in the range of 400-1000nm. The remaining light continues forward through 580 nm interferencefilter 37 to photodetector 38. A separate signal is therefore generatedfor each selected wavelength, and each analog signal generated byphotodetectors 32, 36 and 38 is converted to a digital signal by highspeed A/D converter 24.

The timing signal by which the sampling rate is controlled is 250 kHz.The timing rate is applied to the A/D converter as a conversion ratecontrol signal. This rapid rate of data conversion enables dynamicmeasurement of spacing between the head and disk.

In an alternate embodiment, light beams emitted from two or more laserscan be optically combined to generate the illumination projected ontothe head/disk interface. Such a configuration is illustrated in FIG. 3in which lasers 41 and 42 each emit at different wavelengths. The beamsare expanded by beam expander optics 43 and are combined at beamsplitter44 to be projected through the microscope objective as above. Thedetector assembly 45 will have the same number of detectors as lasers,and the appropriate interference filters for the selected lasingwavelengths will be provided.

The accuracy and repeatability of the location of the head 12 withrespect to the disk 10 requires that appropriate attachment means areprovided to hold the head 12 into place. The suspension 13 must beattached to and removed quickly from the mounting arm 17, and theattachment must be uniform and repeatable. In order to quickly andrepeatably grip the suspension 13, the use of a flexible band 48 ofpolyester film, such as Mylar® or Kapton®, is used, as shown in FIG. 7 ,with a tension means 49 included to keep the band 48 taut. Since thesuspension 13 is somewhat flexible, it may be deformed under uneven orexcessive pressure as might be applied by a steel clamp. The flexibleband 48 does not deform the suspension 13 and is very thin, as requiredby the small spacing between the suspension 13 and the disk 10.

The procedure for measuring the spacing between the head 12 and the disk10 is as follows:

The head 12 is attached to the suspension using the flexible band 48,and the rotary load/unload arm 17 moves the head 12 into the desiredlocation, as in FIG. 8b. Light from the light source 2 is projectedthrough the microscope 3 onto the head 12, and the head 11 is thenpartially unloaded by rotation of arm 17, as illustrated in FIG. 8c, sothat a calibration trace can be generated. The head unload motion issynchronized with the digitization by the processor 26 such that thedigitization will record the intensity of one maximum and one minimumfringe for each individual wavelength tested. The rotary head unload isused so that the slider 12 has only very slight motion while the loadingpressure is reduced. The slight motion of the slider 12 is compensatedfor by moving the microscope assembly 3 by way of x-y translators 5 and5' in synchronization with the unload motion so that the measurementspot remains at the same point on the slider 12. As previouslydescribed, the unload motion required for calibration increases thespacing by a very small amount, on the order of 0.5 μm.

The calibration trace is digitally lowpass filtered to reduce electronicnoise. A maximum and minimum intensity for each color in the calibrationtrace is found by searching through the data collected. The trace isnormalized to the maximum and minimum intensity for each color bymultiplying a constant and adding a constant offset. In the preferredembodiment, the maximum intensity for each other color is scaled to 1.0and the minimum intensity for each color is scaled to -1.0.

A look-up table for intensity of each color versus spacing is generatedfor discrete steps of spacing based on Equation 1 which was derived frommultiple beam interferometric theory for a single film. ##EQU1## where:r=Amplitude Reflection Off Glass Disk:

s=Amplitude Reflectance Off Slider;

n=Refractive Index of the Fluid in the Spacing;

δ=4πn H/λ-γ;

H=Flying Height;

λ=Wavelength of the Illuminating Light; and

γ=Phase Shift on Reflection (Material Dependent)

The values of r, s, and γ are determined by ellipsometric measurement ofthe surfaces using light of the same wavelengths as those being used forthe interference measurements of spacing.

The ellipsometer measures angles ψ and Δ from which the complex index ofrefraction n=n-ik can be determined. The reflectance for the disk andslider can then be determined from Equation 2: ##EQU2## where n₀ is therefractive index of the medium immediately above the reflecting surfacewhich has the refractive index n₁.

For the internal glass reflection n₀ is the index of glass and n₁ is theindex of the fluid in the spacing. For the reflection off the slider, n₀is the index of the fluid in the spacing (air or liquid) and n₁ is theindex of the slider.

From the ellipsometric measurements, the phase change upon reflection γcan be calculated using Equation 3: ##EQU3##

For use in Equation 1, γ=γ_(slider) -γ_(glass) disk.

In the present embodiment, a look-up table is generated using Equations1, 2 and 3 with intensities for spacings for 0 to 1,000 nm with steps of1 nm. The theoretic intensity look-up table is normalized to fringeintensity extrema using the same convention as that used for themeasured traces, i.e., +1 is maximum, -1 is minimum.

Table 1 illustrates the form of the look-up table for intensity versusspacing:

                  TABLE 1                                                         ______________________________________                                                 INTENSITY                                                            SPACING    λ1     λ2                                                                            λ3                                     ______________________________________                                        0          -1            -1     -1                                            5          -0.8          -0.7   -0.6                                          10         -0.6          -0.4   -0.2                                          ______________________________________                                    

Once the measurements are made and the look-up table is generated, acalculation is made to determine the spacing for the first point in thecalibration curve. The calculation of spacing is made as follows:

A "mean square error" is calculated for each discrete step of spacing(H) in the range of interest. Here, steps of spacing from 0 to 1,000 nmin steps of 1 nm are used. These steps conveniently correspond to thevalues in the theoretical look-up table above. The mean square error (e)is calculated as follows:

    e(h)=w.sub.1 (T.sub.1 -M.sub.1).sup.2 +w.sub.2 (T.sub.2 -M.sub.2).sup.2 +w.sub.3 (T.sub.3 -M.sub.3).sub.2                         (4)

where T_(x) is the theoretical intensity of color x for the spacing H(from the look-up table) and M_(x) is the measured intensity of color x.

The symbols w_(x) are used to indicate weighing functions which helpreduce error. While all of the w's can be set to one, accuracy can beimproved by choosing the w's such that w₁ +w₂ +w₃ =some constant andsetting each w higher for a color when the signal-to-noise ratio of thatcolor's intensity measurement is higher. Additionally, w_(x) may be afunction of spacing because, near the maximum or minimum of a givencolor's intensity, a small amount of electrical noise corresponds to arelatively large change in spacing.

For reference purpose, the "first point" of a trace requires intensitymeasurements for all three colors, λ₁, λ₂ and λ₃. The measured intensityof all colors are stored together and they, as a group, will be referredto as a single point in the intensity trace.

Once (e) has been calculated for each step of spacing, the smallestvalue of (e) corresponds to a first guess for the initial spacing forthe first point in the calibration array. Occasionally, noise in theintensity signal may result in the calculated value being the wrongfringe order. For example, considering only one color, if the normalizedintensity is -1, the spacing is given by nλ/2 where λ is the wavelengthof light and n is the fringe order (n=0, 1, 2, 3, 4 . . .). This resultcomes from a simple model which assumes the disk and slider are made ofdielectric materials.

The use of two or more colors gives additional information whichtheoretically indicates the correct fringe order. However, the additionof some noise to the measured signals will occasionally cause thistechnique to calculate an initial spacing which is significantlyincorrect because it is on the wrong fringe order. The principle bywhich fringe order is determined can be explained by looking at a plotof intensity versus spacing for interference fringes generated for twoor more colors. FIG. 10 illustrates such a plot for two colors. Theintensity function 81 for λ₁ has a different period than that for λ₂(82). By comparing the normalized measured intensity with the normalizedtheoretical intensity for both colors the spacing can be determined butthere may be some uncertainly as to fringe order. For example, thenormalized intensity difference between λ₁ λ₂ at points 84 and 85 is thesame at different orders. This is solved by measuring the intensitycontinuously while the spacing between the transparent article and thereflective surface is changed (increased) which shows the slope of therespective functions beyond the initial point 84. This enablesidentification of the correct order.

To determine the correct fringe order, the entire calibration intensitytrace is used as follows;

Several different first guesses are made for the spacing for the firstpoint in the calibration trace. The criteria for selection of the firstpoint initial guesses for spacing are that each spacing must have a lowvalue of (e). Additionally, each guess must be at least 100 nm away fromall the previously-selected initial guesses for spacing. Once several ofthe initial guesses are obtained, one of the guesses is correct, but thecorrect one must be determined.

In order to determine which first guess is correct, a cumulative meansquare error for each initial guess is calculated. The cumulative erroris calculated by assuming that the spacing does not change by more than100 nm between each point. The following process is used to calculatethe cumulative mean square error for each initial guess of spacing.

First, the cumulative error is set to the (e) which corresponds to theinitial guess for the spacing at the first point in the calibrationtrace. Next, the spacing at the second point is calculated by searchingthrough all spacing values within ±100 nm from the first point. The (e)determined for the second point is added to the cumulative error. Thesecond step is repeated for all remaining points in the calibrationtrace with the condition that each point must not be more than 100 nmaway from the previous point. Once the cumulative error is calculatedfor each initial guess for spacing of the first point in thecalibration, the spacing with the lowest cumulative error is determinedto be the spacing for the first point in the calibration.

Analysis of the data taken during calibration has yielded the maximumand minimum intensity for each fringe and the fringe order for thespacing measured during the calibration step.

Since operational parameters may change after the after the calibrationbut before the measurement, the fringe order may change, hence a"follow" trace is recorded while any operation parameters are changed.It is assumed that the first point in the follow trace is within 100 nmof the first point of the calibration trace.

The spacing for each point in the follow trace is calculated as follows:

Point-by-point, the mean square error is determined for spacings withvalues from -100 to +100 nm away from the previous point. Note that,since the first point in the follow trace does not have a previouspoint, the spacing is calculated from the first point in the calibrationtrace. The spacing with the lowest (e) is determined to be the spacing,and the process is repeated for all points in the trace. By limiting thecalculation such that each successive point must be within 100 nm of theprevious point, the calculation can never jump to an incorrect fringeorder.

Once the spacing for the follow trace is calculated, it can be assumedthat the spacing for the first point in the measurement trace is within100 nm of the last point of the follow trace. Since the approximatespacing for the first point of the measurement trace is known, thespacing for the measurement trace is performed using the same procedureused to calculate the spacing for the follow trace.

It should be noted that the 100 nm difference limit is arbitrarilyselected and that other values may be used as allowed by the wavelengthsof light used for measurement, i.e., the limit should not exceed 1/4wavelength.

In the preferred embodiment, the flying height tester measures at least256 points per revolution. A typical test involves storage of data ofintensity versus angle for two revolutions. Thus, at least 512 pointsare taken for each data set. The processor converts intensity versusrevolution data into spacing versus revolution and provides an outputdisplay in either the form of a video monitor or a printout. Since airbearing thickness is a function of disk speed, testing of the spacingbetween a given head and disk is preferably performed at a number ofdisk rotational speeds. FIG. 4 illustrates a series of plots of spacingversus angle for two revolutions. FIG. 4a, b and c show plots at 3,600RPM, 900 RPM and 225 RPM, respectively. FIG. 4d is a plot of spacingversus disk speed illustrating the principle of increased spacing withincreased rotational speed.

FIG. 5 is a plot of spacing versus time to show measurement by thesystem of dynamic variation of head/disk spacing after the head fliesover a flaw in the disk.

Interferometric fringe intensity calibration is used to calibratemaximum and minimum intensity of light and dark interferometric fringes.The maximum and minimum fringe intensity is measured at one or morepoints on the interferometric image while altering interferometric pathlength by at least 1/4 of the light being used to produce theinterferometric image. By comparison of interferometric intensity to themaximum and minimum intensity, it is possible to obtain spatialmeasurement with a resolution of a small fraction of the wavelength oflight used.

While a number of flying height testers use interference to measure thespacing between the head and media, the system of the present inventionutilizes a unique calibration technique in which the head is unloaded ina controlled manner to increase the relative path lengths of the twolight rays while monitoring and storing the intensity information inorder to utilize interferometric fringe intensity for calibration of themeasurement. Thus the need for expensive and difficult control of systemcomponents such as light sources, detector drift and disk quality iseliminated. The apparatus of the present invention provides high speedmeasurement of head/disk spacing simultaneously using multiplewavelengths of light, thus offering a more economical and fastermeasurement system and technique for dynamic measurement of flyingheight than systems that are currently available.

Shown in FIG. 9 is another use of the intensity calibration method, hereused for a micro-hardness tester/surface profiler. In FIG. 9, light fromsource 61 which could be a Mercury arc, a multiple laser, or othermulti-wavelength source, goes through beamsplitter 62, then throughmicroscope objective 63, being focused on the interface betweentransparent probe 64 and the surface to be analyzed 65. The probe islowered onto the surface using computer-controlled actuator 67 whichcould use piezo-electric or other means of mechanically moving the probe64 toward the surface 65. The contact load between the probe 64 and thesurface 65 is detected by a load cell 66. Light reflected from thesurface of the probe 64 which is closest to the sample surface and thesurface itself 65 recombines, producing interference effects.Monochromatic images of the interference fringes are projected onto TVcameras 73, 74, and 75 after going through beamsplitters 62, 86 and 69,as well as narrow band interference filters 70, 71, and 72. Each TVcamera then produces an analog video signal corresponding to theinterferometric image for the narrow band of light associated with eachinterference filter. Note that filters 70, 71, and 72 each transmit adifferent color of light. The analog video signals are directed into theRGB color frame grabber 76 which is under control of the personalcomputer 77.

To make a surface roughness and hardness measurement, the computercontrolled actuator 67 lowers the probe 64 onto the surface 65. Duringthe time that the probe is coming into contact with the surface, theframe grabber 76 continuously acquires images of the three differentinterference images and the computer processes such images, storing themaximum and minimum intensity of each color at each pixel on each image.Before the probe 64 comes into contact with the surface 65, a minimumand maximum intensity of the fringes for each color will have beendetected for each pixel of each image of the fringes. These maximum andminimum fringe intensity values are the normalization intensity valuesrequired to determine spacing between the probe 64 and the surface 65.The spacing between the probe 64 and surface 65 is calculated in amanner identical to that described for the Dynamic Flying Height Tester,except the spacing is calculated for a 2-dimensional area rather than asingle spot, and the spacing is measured statically, not dynamically.Since the probe 64 shape is known beforehand, the surface profile of thesurface 65 can be readily calculated from the spacing between the probe64 and the surface 65. This surface profile indicates the roughness ofthe surface.

The surface hardness and other surface properties can be measured bypressing the probe 64 onto the surface 65. The force with which theprobe is being pressed onto the surface is measured using load cell 66.The surface profile can continue to be measured interferometricallywhile it is being deformed by the probe. The measurement of surfacedeformation as a function of contact pressure will allow a measurementof surface hardness while causing a minimum amount of damage to thesurface.

Both the Dynamic Flying Height Tester and the InterferometricMicro-hardness Tester illustrate ways that the Method to calibrateintensity for interferometric measurement of small spacing can be used.The feasibility of the measurement has been demonstrated by themeasurements of flying height illustrated herein.

It will be evident that there are additional embodiments which are notillustrated above but which are clearly within the scope and spirit ofthe present invention. The above description and drawings are thereforeintended to be exemplary only and the scope of the invention is to belimited solely by the appended claims.

I claim:
 1. A method for simultaneously measuring roughness and hardnessof a surface which comprises:splitting light having a plurality ofwavelengths λ into a first portion and a second portion; directing saidfirst portion along a light path toward an interface between atransparent probe and said surface at a first location of said surface;attaching a load cell to said probe; reflecting said first portion fromsaid interface; combining the reflected said first portion and saidsecond portion to generate fringe patterns; storing maximum and minimumintensity of said fringe patterns for each wavelength λ; calculatingspacing using said maximum and minimum intensity; moving said probetoward said surface while continuously generating said fringe patternsand storing their maximum and minimum intensity; and detecting a contactload between said probe and said surface while said probe deforms saidsurface.
 2. An interferometric apparatus for simultaneously measuringroughness and hardness of a first surface comprising:a light source forproducing light having a first wavelength and a second wavelength; ameasuring probe positioned proximate to the first surface, saidmeasuring probe having a surface adjacent the first surface, saidmeasuring probe surface and the first surface defining aprobe-to-surface spacing having a separation distance; optics fordirecting said first and second wavelengths of light to saidprobe-to-surface spacing having said separation distance, such thatlight reflected from the first surface and said measuring probe surfaceproduces a first wavelength interference signal and a second wavelengthinterference signal, said first and second wavelength interferencesignals having characteristics determined by said probe-to-surfacespacing separation distance; a detector for measuring the intensities ofsaid first and second wavelength interference signals; a calibrationcontroller for changing said probe-to-surface spacing separationdistance while acquiring first and second calibration fringe patternscorresponding to said first and second wavelengths; a processor foranalyzing said measured intensities of said first and second wavelengthinterference signals and deriving therefrom said probe-to-surfacespacing separation distance; and a load cell attached to said measuringprobe wherein said load cell detects a contact load between saidmeasuring probe and the first surface when said measuring probe contactsand deforms the first surface.
 3. An interferometric apparatus asdefined in claim 2 wherein said processor further comprises:alook-up-table which contains a first set of theoretical interferencesignal intensity data which corresponds to said first wavelength oflight and a second set of theoretical interference signal intensity datawhich corresponds to said second wavelength of light; and a comparatorfor comparing said measured intensities with said look-up-tabletheoretical intensities to determine said spacing.
 4. An interferometricapparatus as defined in claim 2 wherein said light source furthercomprises a mercury arc lamp.
 5. An interferometric apparatus as definedin claim 2 wherein said light source further comprises a laser.
 6. Aninterferometric apparatus as defined in claim 2 wherein said opticsfurther comprises a microscope assembly.
 7. An interferometric apparatusas defined in claim 2 wherein said detector further comprises one ofeither a photodetector or a photodetector array.
 8. An interferometricapparatus as defined in claim 2 wherein said light source furthercomprises a first laser for producing said first wavelength of light anda second laser for producing said second wavelength of light.
 9. Amethod for simultaneously measuring roughness and hardness of a surfacecomprising the steps of:positioning a probe adjacent to a surface to bemeasured such that said probe and said surface define a separationdistance between said probe and said surface; measuring said separationdistance between said probe and said surface interferometrically; movingsaid probe toward said surface until said probe contacts and deformssaid surface; and detecting a contact load between said probe and saidsurface while said probe deforms said surface.